How long is a piece of string? A study of line bisection in a case of visual neglect.

We report a case of severe left visual neglect consequent upon extensive infarction in the territory of the right middle cerebral artery. A detailed analysis is given of the patient's performance on line bisection. The stimuli cover a wider range of lengths than is usually employed in such studies (11'' to 1''). The magnitude of the patient's rightward displacement of his transections is linearly related to the length of the stimulus line, such that the longer the line, the greater the left 'neglect'. At line length 2'', the patient's transections are relatively accurate, and at length 1'', the transections cross the objective midpoint to give a reliable left displacement (= 'right neglect'). On some occasions, these latter transections are placed beyond the leftmost point of the stimulus line. We interpret the data in terms of two constructs: (a) an 'attentional boundary' placed slightly to the left of the objective midline; (b) 'representational completion' that extends to the attentional boundary.